1. Field of the Invention
The present invention relates to motor vehicles having a mechanical drive train, and internal combustion engine and an automated friction clutch, and more particularly, to tow-starting and towing away such a motor vehicle.
2. Description of the Related Art
It can sometimes happen that the vehicle cannot be started because of a functional failure of its starter motor. In a conventional vehicle with a foot-operated clutch, it is nevertheless possible for the engine to start if, with the vehicle rolling and a gear engaged, the clutch is engaged by the driver, thus producing a closed torque-transmitting path between the driven wheels of the vehicle and the internal combustion engine. In this case, in order to set the vehicle in motion, the driver can let the vehicle roll downhill without external aid, or have the vehicle pulled by another vehicle. Sometimes, the engine dies as the vehicle is being driven, for example during a gear change. In this case, as well, the driver can restart the engine by engaging the clutch if the vehicle has sufficient speed. The term "tow-starting" refers within the scope of the invention to all situations in which the rolling moment of the driven wheels of the vehicle is to be used to start the switched-off engine by closing the torque-transmitting path to the driven wheels. In this case, switched off means that the engine is not firing, which can be the case after the vehicle has been deliberately taken out of service, but also after the engine has inadvertently died.
After a breakdown or an accident, the vehicle is, however, occasionally not capable of being driven. There is then a need for external aid in order to tow it away. When the vehicle rolls on its driven wheels when being towed away, it is expedient to interrupt the transmission of torque between the driven wheels and the engine in order to avoid damaging the drive train components. This can be performed, for example, by putting the transmission into a neutral position in which no gear is engaged. However, it is sometimes not possible after an accident to put the transmission into its neutral position, for example when the vehicle remains with a gear engaged and the accident has caused damage to the gearbox. In such a case, with the clutch engaged there would be a risk of damaging the engine if the vehicle is towed away at a speed for which the gear transmission ratio set at the moment the gearbox was damaged is not designed and, consequently, the crankshaft of the engine is turned at an impermissibly high speed. In order to eliminate this risk, the driver would then have to release the clutch by treading on the clutch pedal.
In the case of vehicles with an automatic clutch, however, the driver generally has no clutch pedal available for influencing the clutch. If, with the engine switched off and a gear engaged, the vehicle is pulled by another vehicle, the electronic system of the vehicle which controls the clutch is generally unable per se to detect whether it is desired to tow-start the vehicle engine, or whether it is desired to tow the vehicle away (e.g., due to the damaged gearbox). It would be recommendable for safety reasons to release the clutch in order to reliably be able to prevent possible damage which could arise during towing away with the clutch engaged. Starting the internal combustion engine via the drive train would then, not be possible. On the other hand, if the clutch were to be held in its engaged position by the electronic control system, it will be possible to start the internal combustion engine, but in a tow-away situation it would not be possible to rule out damage to the drive train components.